A rotating body such as a flywheel of a flywheel energy storage system, a rotor of a helicopter, a turbine blade, etc. plays an essential role in a mechanical apparatus. Accordingly, it is very important to prevent unexpected accidents by constantly monitoring the rotating body and detecting any indication of breakdown in advance. However, in order to accurately monitor the rotating body during its operation, sensors must be directly mounted to the rotating body. In such a case, there is a problem in that it is difficult to supply power to the sensors and transmit the detected signals outwardly.
As one of the currently used methods for monitoring a rotating body, there is a method using a slip ring, which collects cables of the sensors mounted to the rotating body and connects them like a brush of a DC motor. However, there is a problem with such a method in that each cable must be connected individually. Further, since a rotating member and a fixed member must be successively contacted to each other, other problems related to wear, noise, maximum speed restriction of the rotating body and the like take place due to such contact. Moreover, if the rotating body has a plurality of points to be monitored such as a turbine, then the installation and wiring of the slip ring become highly limited.
There is another method wherein a signal processing device and a battery (in addition to the sensors) are integrated in the rotating body and become wirelessly connected to the outside. However, the battery must be periodically replaced. Further, mounting the above monitoring equipment to the high-speed rotating body can be extremely difficult due to the weight and size of the equipment.
Further, considering such a problem, an indirect measuring method is used wherein the rotating body is monitored by using the sensors equipped outside the rotating body. In this method, vibration is measured and analyzed by using an accelerometer, which is mounted to a housing of a bearing, or a gap sensor that detects the vibration of the rotating body. However, since it is very difficult to show a link between the vibration signals and the indications of defect or breakdown of the rotating body and to further detect the abnormalities of the rotating body at an initial step in the above indirect monitoring method, regular maintenances considering numerous safety factors are performed at many fields. Thus, there is a problem in that an enormous expense and manpower are needed due to the excessive maintenance.
As a solution for the above-mentioned problems, Korean Patent Publication No. 10-2005-109191 (filed on May 14, 2004 by the present applicant) discloses a monitoring device for a rotating body comprising the following: a light source for emitting a multi-wavelength light; an optical fiber connected to the light source and being extended to oppose a center of a rotating shaft at its one end; a fiber Bragg grating sensor mounted to the rotating body, the fiber Bragg grating sensor being positioned so that its one end is located at a center of the one end of the rotating shaft and is opposed to the one end of the optical fiber, the fiber Bragg grating sensor reflecting light with wavelength corresponding to deformation of the rotating body in response to the light transmitted via the optical fiber; and a signal processing unit connected to the optical fiber, the signal processing unit receiving the light reflected from the fiber Bragg grating sensor via the optical fiber and calculating the deformation of the rotating body based upon the reflected light.
However, there are two problems with the prior art monitoring device for a rotating body. One problem is that transmission loss, which necessarily occurs when light signals move between a rotating body and a fixed body via a space, is varied in intensity depending on a rotation angle. If the transmission loss of the light signals is constant irrespective of the rotation angle, then no problems occur. However, it is very difficult to mount the collimating and focusing members to the rotation-side and the fixed-side, respectively, so that their centers are exactly in a line. Further, it is very difficult to make the collimating and focusing members to transmit completely parallel light signal between the rotation-side and the fixed-side. Therefore, rotation of the rotating shaft changes the transmission loss of the light signals between the rotation-side and the fixed-side according to the rotation angle, thereby highly distorting signals from the Bragg grating sensors mounted to the rotating body.
FIGS. 10A and 10B are graphs of a total light quantity, which shows the transmission loss variation of the signals from the Bragg grating sensors according to the rotation angle variation of the rotating body. The total light quantity is obtained by measuring the light signals reflected from each sensor in the rotating body at the fixed-side after one optical fiber with a plurality of Bragg grating sensors is mounted to the rotating body and a broadband signal is then transmitted from the fixed-side to the rotation-side. FIG. 10A shows the transmission loss variation of the light signals according to the rotation angle of 0°˜360° as an x-y graph. FIG. 10B shows the transmission loss variation of the light signals according to the rotation angle of 0°˜360° as an angular graph. Since the transmission loss of the light signals is constant irrespective of the rotation angle in case of ideal connection, y-axis values of FIG. 10A must be constant and a circle must be made in FIG. 10B. However, it can be seen that the light quantity is highly varied according to the variation of the rotation angle. The graphs of FIGS. 10A and 10B show that the total light quantity reflected from the sensors is measured. However, separately measuring the intensity of the light of the specific wavelength reflected from each sensor also shows the same pattern of the variation of the transmission loss as the above graphs. When measuring strain by means of optical sensors, the intensity of the specific wavelength (light quantity) must be measured. Since the variation of the light quantity according to the rotation angle of the rotating body is high as described above, it must be certainly compensated.
The other problem is that temperature must be accurately measured considering the characteristic of the optical fiber (i.e., wavelengths reflected from the Bragg grating sensors are varied according to temperature) and the characteristic change according to the temperature variation must be compensated. The wavelength variation of the Bragg grating sensor is sensitive to temperature as well as strain. Therefore, there is a need to accurately measure the temperature of the sensor portion and to calculate the wavelength variation caused by the temperature as well as to compensate it in order to calculate the accurate strain.